Increased arsenic mobilization in the rice rhizosphere is mediated by iron-reducing bacteria

Rice (Oryza sativa) tends to accumulate elevated levels of arsenic (As) in grain, threatening food safety and human health. The rice rhizosphere has a micro-environment that differs markedly from the bulk soil. Yet, little is known about how this micro-environment influences the mobility of As in the rhizosphere. Using rhizoboxes with two rice cultivars (cv. Shenyou 957 and Yangdao 6) differing in their radial oxygen loss (ROL), we investigated the in situ transformation of As in the rhizosphere associated with changes in microbial communities and As-related functional genes. Contrary to expectation, dissolved (porewater) As concentrations within the rhizosphere increased by 1.3-2.4 fold compared to the bulk soil during the seedling stage, with the magnitude of this difference gradually decreasing over time. The increased As mobilization in the rhizosphere was associated with increased soluble Fe. This increasing trend was associated with the increased abundance of both Fe-reducing bacteria (FeRB) and As-related functional genes within the rhizosphere. Furthermore, bacterial 16S rRNA gene sequencing data showed that the abundances of Geobacter and Clostridium were 3.1 times and 12.4 times higher in the rhizosphere, respectively. The importance of FeRB was also suggested by the fact that dissolved As concentrations were highly correlated with dissolved Fe concentrations (r² = 0.83) and also with the relative abundance of genus Clostridium_sensu_stricto_10 (r² = 0.85). This study highlights that although the rice rhizosphere favors a more aerobic condition compared to the bulk soil, As is more mobilized in the rhizosphere, and that Geobacter and some species of Clostridium play a critical role in controlling As mobilization in the rhizosphere.